1. Field of the Invention
The present invention relates to a device for reading information from an optical disk on which information can be recorded, such as a WORM (Write Once Read Many) optical disk, and more particularly to a track crossing detector for detecting a traversing or crossing motion across a track for tracking searching operation.
2. Description of the Prior Art
Information storage optical disks include a WORM optical disks on which information can be recorded only once. Various mechanisms have been proposed to record desired information on a WORM optical disk. One optical disk with a new information recording mechanism is known as a dye optical disk. When a laser beam is applied to the dye optical disk, a disk region where the laser beam spot is applied is discolored or deformed to provide a different reflectance from the reflectance of another disk region. Therefore, the discolored or deformed disk region functions as a signal pit. Optical disk of this kind a narrow land on its surface which serves as information tracks, which will be described in detail with reference to FIG. 1 of the accompanying drawings.
As shown in FIG. 1, a WORM optical disk 9 has a narrow spiral land 10 on its recording surface, the spiral land 10 being spirally extending from the inner circumferential side toward the outer circumferential side of the disk. The recording surface of the optical disk 9 also includes a spiral groove 11 defined between successive turns of the spiral land 10, and, as a result, has a zigzag-shaped cross section in the radial direction of the optical disk 9. The recording surface with the land 10 and the groove 11 is given a mirror finish for a higher reflectance with respect to the applied laser beam. The land 10 has, on its upper surface, recorded pits 12 alternating with normal surface regions 13, the recorded pits 12 having a lower reflectance than that of the normal surface regions 13. Therefore, the successive turns of the spiral land 10 with the recorded pits 12 serve as information tracks.
The information that is recorded in the form of the recorded pits 12 on the information tracks of the WORM optical disk 9 can be read by an information reading device. The information reading device has an optical pickup (not shown) for optically reading the recorded pits 12 based on the intensity of a reflected laser beam from the optical disk 9, and an RF detector circuit for detecting the envelope of a read signal (hereinafter referred to as an "RF signal") and supplying the detected envelope to a demodulator. The RF detector circuit has a track crossing detector for searching the recorded information at high speed. One conventional form of track crossing detector is illustrated in FIG. 2.
When the optical pickup moves at high speed radially over the optical disk 9, i.e., jumps over the tracks of the optical disk 9, the track crossing detector counts amplitude variations of the RF signal due to the alternating occurrence of the land 10 and the groove 11 thereby to determine the number of information tracks that the optical pickup has traversed. As shown in FIG. 2, the track crossing detector has a peak level detector 1 for detecting the peak level of the amplitude of the RF signal, denoted at A. The peak level detector 1 has an output terminal connected through a coupling capacitor 2 to an amplifier 3 that amplifies a detected peak signal B from the peak level detector 1. The coupling capacitor 2 serves to cut off a DC component contained in the peak level signal B and passes only an AC component contained in the peak level signal B. The amplifier 3 has an output terminal coupled to an inverting input terminal of a comparator 4. A reference voltage V.sub.REF is applied to a noninverting input terminal of the comparator 4. The reference voltage V.sub.REF may be a ground potential, for example. The comparator 4 produces an on-track signal C indicative of track count pulses at its output terminal.
Operation of the track crossing detector shown in FIG. 2 will be described below with reference to FIGS. 3(A) through 3(C). During normal reading operation, the optical pickup traces the land 10 while the optical disk 9 is rotating at a predetermined speed. The optical pickup applies an RF signal A whose envelope is substantially uniform, as shown in FIG. 3(A), to the peak level detector 1. As a result, the peak level detector 1 produces a constant detected peak signal B as shown in FIG. 3(B), and the comparator 4 produces a constant on-track signal C as shown in FIG. 3(C). During track searching operation, the optical pickup moves at a certain speed radially over the optical disk 9. At this time, the optical pickup produces an RF signal A whose envelope varies depending on the alternating occurrence of the land 10 and the groove 11, as shown in FIG. 3(A). The envelope of the RF signal A varies because the intensity of the reflected laser beam varies as the optical pickup travels alternately across the land 10 and the groove 11. Consequently, the peak level detector 1 detects successive peak levels of the RF signal A, and generates a detected peak signal B indicative of the varying envelope, as shown in FIG. 3(B). The detected peak signal B is applied through the coupling capacitor 2 and the amplifier 3 to the comparator 4. The comparator 4 compares the detected peak signal B with the reference voltage V.sub.REF. With the reference voltage V.sub.REF being selected at a suitable level, the comparator 4 produces at its output terminal an on-track signal C that represents pulses corresponding to the respective amplitude variations of the detected peak signal B. Since the pulses of the on-track signal C correspond respectively to the turns of the land 10, i.e., the information tracks traversed by the optical pickup, the number of information tracks that the optical pickup has jumped over can be determined by counting the pulses of the on-track signal C. The above description applies to the detection of information tracks on which information has already been recorded, typically on ordinary optical disks, such as CD, LVD, etc.
Information may not necessarily have been recorded on some information tracks on the WORM optical disk 9. For example, as shown in FIG. 1, the recorded pits 12 are formed on some information tracks, and no recorded pits are formed on other information tracks. The WORM optical disk 9, therefore, contains a recorded regions AW and an unrecorded region NW. In FIGS. 4(A) and 4(B), during normal reading operation, the track crossing detector operates in the same manner as shown in FIGS. 3(A) and 3(B). During track searching operation, however, the RF signal A and the detected peak signal B shown in FIGS. 4(A) and 4(B) have different waveforms from those shown in FIGS. 3(A) and 3(B). More specifically, insofar as the optical pickup jumps over information tracks in the recorded region AW, the RF signal A shown in FIG. 4(A) is the same as the RF signal A shown in FIG. 3(A). However, when the optical pickup starts to jump over information tracks in the unrecorded region NW, the signal produced by the optical pickup does not contain any RF signal component from the boundary between the recorded and unrecorded regions AW, NW because no recording bits 12 exist on the land 10 in the unrecorded recorded region NW. The detected peaked signal B has different amplitudes on opposite sides of the boundary between the recorded and unrecorded regions AW, NW, as shown in FIG. 4(B), since the envelope amplitude does not vary in the unrecorded region NW as the signal from the optical pickup has no bottom level (negative peak level) in the unrecorded region NW. As a result, the detected peak signal B in the recorded region AW and the detected peak signal B in the unrecorded region NW have different peak levels which differ from each other by a step Vds. Because of the step Vds, the detected peak signal B in the recorded region AW and the detected peak signal B in the unrecorded region NW have different DC levels, respectively. The different DC levels cause the comparator 4 to fail to produce a correct on-track signal C when it compares the detected peak signal B in the recorded region AW and the detected peak signal B in the unrecorded region NW with the reference voltage V.sub.REF, resulting in an miscount of information tracks traversed by the optical pickup. Therefore, it is necessary that DC component contained in the detected peak signal be quickly absorbed or removed at the boundary between the recorded and unrecorded regions AW, NW.